The idea for this book emerged from various discussions between the two editors about the ways in which criminal justice policy emerges, takes shape and is implemented through the activities of practitioners on the ground. The purpose of the collection of essays is to explore a number of related themes within policy change in criminal justice. The subtitle ‘Who Knows Best?’ is meant to stimulate discussion about policy-making and its implementation (or not) through practice. How and why do particular criminal justice policies emerge from the political process and what are the contributions of politicians, civil servants, practitioners, researchers and others in the generation of those ideas? What is the relationship between the increasingly centralised formation of policy in Whitehall and its local implementation and delivery? To what extent is centralised policy interpreted and refined differently in local areas? Does diversity in implementation imply policy failure, or is it a sign of healthy activism among local practitioner groups? What importance does local justice have? When can the centre learn from local initiatives? We invited contributors to write chapters on topics of particular interest to them, but to consider while doing so the aims, merits and limits of the ‘top-down’ approach to criminal justice policy-making and the involvement of policy-makers and practitioners in the management of change. The authors are well placed to offer a range of perspectives on these issues, whether through their own involvement as policy-makers, or practitioners, or campaigners or as academic researchers and writers. All approaches are represented here. Some of the essays reflect upon policy developments within particular historical periods (such as criminal justice policy under Thatcher, the implementation of community service orders in the 1970s and youth justice practitioner experiences in the 1980s), or in particular parts of the country (community justice in Scotland and youth justice in Wales) and some deal with contentious contemporary policy (such as ‘transforming rehabilitation’ and payment by results, multi-agency work on prolific offenders and proposed reforms to youth courts).